This invention relates to glass microbubbles and is especially concerned with bubbles having a novel composition.
Glass microbubbles have been known for many years, as is shown by U.S. Pat. Nos. 2,978,340, 3,030,215, 3,129,086 and 3,230,064, all of which teach a process of manufacture involving simultaneous fusion of the glass-forming components and expansion of the fused mass; these bubbles, however, are extremely water-sensitive and may disintegrate when incorporated in aqueous compositions, e.g., wall repair compounds. U.S. Pat. No. 3,365,315 discloses the formation of water-resistant discrete free-flowing masses of glass bubbles by "blowing", or expanding, amorphous glass frit. This patent discloses an extremely broad range of possible components, embracing almost all conventional glasses. U.S. Pat. No. 4,391,646 also discloses the formation of glass bubbles from frit, the resultant product consisting essentially of 60-90% SiO.sub.2, 2-20% alkali metal oxide, 1-30% B.sub.2 O.sub.3, 0.005-0.5% sulfur (0.0125-1.25% if calculated as SO.sub.3) and other conventional glass-forming ingredients. Glass microbubbles made in accordance with these patents have achieved wide industrial use, but the inclusion of such raw materials as lithium carbonate, potassium carbonate, and sodium pyrophosphate in the raw materials used to prepare the frit from which the commercial product is obtained, increase cost.
Japanese laid-open application A 49-37565 discloses 6 glass compositions, all of which have conventional ratios of alkaline earth metal oxides to alkali metal oxides (i.e., 1.0 or less). This patent also teaches that, to avoid excessively high viscosities and devitrification, at least 10.0% Na.sub.2 O or 13.0% (Na.sub.2 O plus K.sub.2 O) should be included.
European Patent Application No. 165875 discloses that using a reducing atmosphere when flame-forming the glass bubbles, particularly for very small feed particles of less than 20 micrometers, increases the effectiveness of the sulfate blowing agent. All examples, however, have conventional ratios of alkaline earth metal oxide to alkali metal oxides.